Manufacture of filaments



Sept. 15, 1942. R. T, FIELDS 2,295,942

MANUFACTURE OF FILAMENTS Filed Aug. 2, 1940 Reuben T Fields INVENTCR BY I41. 04m

ATTORNEY Patented Sept 15,

,U Nl TED I MANUFACTURE. DEFILAMENTS, Reuben T. Fields, ArlingtomN. .L, assignor to E. I.

du Pont de Ncmours & Company, Wilmington,

DeL, a corporation 02 Delaware Application August 2, 1940, Serial No. 349,840

5 Claims.

7 This invention relates to the manufacture of filaments and, more particularly, to the manuiacture of relatively coarse filaments of synthetic linear condensation polymers by melting and spinning the polymers.

By the term synthetic linear condensation polymers is meant new polymeric materials such as described in United States Patents 2,071,250 and- 2,071,253, and in United States application Serial No. 136,031, filed April 9, 1937, now Patent No. 2,130,948, all of W. H. Carothers.

These synthetic linear condensation polymers are subject to destructive change at the elevated temperatures at which they become molten. A

tendency to undesired further polymerization during the steps of melting and spinning is minimized by limiting the dimensions of the equipment used in these steps and thereby limiting the duration of exposure oi the material to high temperature. high temperature is a slow chemical decomposition, which results in the release of gaseous products.

It has been recognized, in connection with any and all spinning of filaments of these materials, that the presence of bubbles of gas, coming from decomposition, or from simple entrapment from A further result of exposure to the atmosphere of the vessel-in which the polymer is melted, isdefinitely objectionable since, if such bubbles are carried through to the spinneret, they are likely to result in broken or defective filaments, or otherwise cause irregularities in the operation of spinning. In a copending application of Graves, Serial No. 232,314, filed September 29, 1938, now Patent No. 2,278,875, provision posed upon the molten polymer to overcome the.

frictional resistance of the spinneret and its accessorles to the spinning of fine filament at commercial rates, and also because the smallness of the diameter of such filament permits it to be so rapidly hardened by chilling, as it emerges from the spinneretythat gases dissolved in it are not released, and no bubbles are formed.

In the manufacture of coarse filaments, such as those used as 'monofilaments for bristles,sutures, fish-line leaders, racquet strings, et cetera, the specific problem is rather difieren't. Filaments of the much larger diameters here involved, of the order of 0.020 inch (undrawn) and upward, and particularly of 0.040 (undrawn) inch and upward, are so much less rapidly chilled through, after emerging from the spinneret, that gases previously dissolved in the molten mass by application'of high pressure have opportunity to become released during the time between the release of the pressure at the exit of the spinneret and the effective solidification of the filament.

Particularly troublesome is water dissolved in the melt as a result of its presence in the initial polymer and of a failure to expel it from the molten mass.

Broadly, it is the object of the present invention to provide equipment and procedure whereby to spin, from synthetic linear condensation polymers, coarse filaments of uniform diameter and free from bubbles and discoloration.

More specific objects are to provide equipment and procedure to eiTect melting of the polymers without access of oxygen, to minimize the duration of exposure of the molten material to temperatures capable of causing its decomposition, to deliver molten material bubble-free to the spinneret under uniform pressure, and to conduct the process as a whole, from melting to spinning, under minimum pressure, in order to minimize the content of releasable gases within the molten material.

A further object of the invention is to provide equipment which is adapted to handle molten polymer over a wide range of consistency and over a wide range of initial admixture with gas, and to provide so effectively for the escape of water vapor that it is adapted to handle initially wet polymer. Other objects will be apparent from the description or the invention given hereinafter.

will be more fully discussed hereinafter.

The above objects are accomplished according to the present invention by a process which comprises melting a synthetic linear condensation polymer while blanketed with carbon dioxide to the exclusion of oxygen and at no greater than atmospheric pressure, introducing the melted polymer into a screw pump having a capacity at its intake end several times greater than its capacity at its discharge .end, and delivering said molten polymer under pressure to a spinneret to cause the molten polymer to pass through the orifice of the spinneret and emerge as a filament, the polymer being kept out of contact with oxygen from the time it is melted until its emergence from the spinneret as a filament.

The melted polymer may be forced through the spinneret either by the pressure developed in the screw pump or by auxiliary pumping means interposed between the screw pump and the spinneret. The fllame t should be rapidly cooled as it is extruded a this is preferably accomplished by bringing it into contact with water.

, By "capacity" of the screw pump is meant the volume of material forwarded per turn of the screw, assuming geometrically perfect delivery with no compression or slippage. To carry out the present invention the capacity of the screw pump must be several times as great at its intake end as at its delivery end, preferably in a ratio of not less than 4 to 1. Although the screw pump may be constructed so that its change in capacity from end to end is gradual, it is usually more convenient to construct the screw pump so that the capacity is changed abruptly at some point alongdvhe Rrew and this type of screw pump is preferred.

In the preferred form of apparatus according to the present invention, the screw pump is used to deliver the molten polymer directly to the spinneret without any interposed auxiliary pumping means. In such apparatus, not only is the ratio of the capacity of the screw pump at its intake end and delivery end important but, also, it is important that the depth of the flute of the screw, at the delivery end particularly, should not condensation polymer is melted in a chamber I I which is desirably of cylindrical shape with a truncated conical bottom and is provided with a cover I2 containing a port I3 through which the polymer may be fed into the chamber. In the lower part of the chamber II are banks of tubes I, I4 running at right angles to each other and adapted to carry a heating fluid for melting the polymer. The cylindrical sides and coni- 1 cal lower portion of the chamber are enclosed in a Jacket l-5 similarly adapted to carry a heating fluid in circulation, and the tubes I4, I4 preferably communicate with this jacket so that a single source of heating fluid will provide the circulation thereof in .both Jacket and tubes.

The heating chamber II is provided also with means for the entry and escape of the blanketing gas. Conveniently, these take the form of pipes passing through the cover I2 and, preferably, the intake pipe I6 extends downward within the chamber nearly to the level of the uppermost of the banks of tubes while the outlet pipe I1 is located near or in communication with the port I3. Throughout the operation of the equipment, the blanketing gas, carbon dioxide, is fed into the chamber II through the pipe I6 and flows out of the chamber either through the pipe I! or, if the port I3 is open, then also through this port. By this means, air is excluded from the interior of the chamber I I while it is in operation and the polymer is protected against contact with air throughout the duration of its stay in this chamber without using superatmospheric presexceed .150" in any screw pump of suitable size I and proportions for use for this purpose. This y depthoithe flute" is meant the distance, measured radially, from the floor of the flute in the screw to the inner surface of the casing of the screw pump. Practically, the depth of this flute should bebetween .050" and .150". If an un-' usually viscous melt were to be pumped, the depth of the flute might be slightly greater.

The present invention resides in the discovery that proceeding as described above coarse filaments of .010" diameter and greater, especially those filaments of- .020? diameter and greater, undrawn, may be readily extruded without flaws whereas applicant has been unable to find any other practical way of manufacturing such coarse,

filaments without encountering flaws therein, presumably caused by the release of gas dissolved in the polymer prior to its extrusion. The procedure of the present invention is carefully designed to reduce the gases absorbed by the molten polymer to such a minimum that they are not released from the extruded filament.

In the drawing forming a part of the present application, the figure of the drawing is a vertical section, more or less diagrammatical, of an apparatus constructed according to the preferred form of this invention.

Referring to the drawing, the synthetic linear sure. Water vapor also escapes with the carbon dioxide.

While the constant escape of blanketing gas can be relied upon to prevent the entry of air into the chamber I I through the port I3 when the latter is open, it is desirable that the feeding port I3 be provided with a cover I8 and that this be kept in place whenever the port is not being used for feeding. When this cover I8 is in place, the

.blanketing gas escapes through the outlet pipe I I. The blanketing gas is fed into the chamber I I merely at a rate which is fast enough to insure against the entry of air through the port I3, the discharge pipe IT, or any other possible point of entry.

An opening at the bottom of the truncated conical portion of the chamber II communicates with an opening into the casing I9 of a screw pump of special design. This casing I9 is provided with a jacket 20 for the circulation of a heating fluid, this jacket 20 and the jacket I5 of the heating chamber intercommunicating through a duct 2|. This closed circulating system is provided with an inlet 22 in the jacket t5 and with an outlet 23 in the jacket 20 for return of the heating fluid to the source of heat.

The interior wall of the casing I 9 of the screw .pump is accurately cylindrical. Within it, with clearance as hereinafter described, rotates the screw member 24 which, at the intake end of the screw pump, is extended through the casing I9 to constitute a shaft 25 by which the screw member 24 may be rotated through a chain drive actuating a pulley or sprocket 26 keyed to this shaft 25.

Within the casing I9, at the intake end, the screw member 24 carries a double-lead flute 21 which terminates in a circumferential groove 28.

From this point on, to the discharge end of the screw pump at 29, the screw member 24 carries a single-lead flute having a capacity substantial- 1y less than that of the double-lead flute'Z'I.

. The dimensions of the screw pump may suitably be as follows:

, Inches Total length of screw Length of screw with double-lead flute" 4 Length of screw with single-lead flute--- 11 '.Rotation of the screw member 24 serves to convey molten polymer, falling from the surfaces of the tubes l4, l4 into the casing of the screw pump, from the intake end of the latter to the discharge end at 29. The manner of operation of this screw, member 24 will be more completely described hereinafter. I

The discharge end of the pump at 29 com- 'munica'tes, through suitable channels, with a spinneret 30 through which, by the pressure the diameter of the filaments produced by a spinneret of given diameter of orifice. The important influences on the diameter are the linear rate at which the filament produced is carried away from the spinneret and the rate of rotation of the pump screw.

It will be understood that the design or the screw pump as illustrated in the drawing and Depth of double-lead flute 0.125 Width, of double-lead flute 0.250 Pitchofdouble-lead flute 0.667 'Depthof'si'ngle-lead flute. 0.063 t Width of single-lead flute; -0.250 I Pitch of single-lead flute 0.333- Inside diameter of easing l 1.625 Clearance between screw and casing,

about 0.0025

- polymer, which is a viscous liquid, may be likened to rolling a pencil between the palms of the hands with one palm held stationary. That is,

. the molten polymer is, in effect, rolled between generated, as hereinafter described, by the screw member 24 within the casing 19, the molten polymer is extruded to form filaments and the like.

As shown in the drawing, the channel communicating with the spinneret is contained within a block 3! attached to the casing I! of the screw pump at the discharge end. A hole drilled through this block, parallel with the axis of the screw member 24, forms a channel 32 which at its entrance end is in communication with the interior of the screw pump and which at its opposite end is in communication, through a diaphragm, with a pressure gauge 33.

A second hole cut in the block 3 I, conveniently at right angles with the channel 32 and not extending through the block, constitutes a channel 34 which serves to connect the channel 32 with the spinneret 30.

Molten polymer enters the casing IQ of the screw pump from theheating chamber II and is carried along in the direction of the spinneret by the rotation of the screw member 24.

It is preferred to rotate the screw member 24 at about 350 R. P. M. although it may be rotated in a ratio of at least 4 to 1.

When the entire system, comprising the screw pump, the channels 32 and 34, and the spinneret fixture 30, has become filled with molten polymer,

the propulsive and compressive action of the the rate at which the molten material is ex-- truded and constitutes a minor influence upon .of what the diameter may be.

the stationary inner surface of the pump casing and the'moving floor of the flute in the screw. 3 From this analogy, it will be apparent why the U a depth of the flute is such an important factor in the design of the pump. The depth of the flute should be between .050" and .150".

The side walls of the flute offer resistance to the desired motion of the molten polymer and, for this reason, the flute must not be too narrow; it has been found that the width of the flute should not be less than about 0.125 inch, and preferablynot less than about 0.25 inch.

On the other hand, the width of the flute should not be too great, since on a screw of reasonable length the path of the flute would be shortened thereby too much to prevent back eter of the screw, and it has been found to be a desirable rule that the width of the flute shall not exceed one third of the diameter, regardless At a fixed ratio of width of flute to diameter of screw, a change in the latter does not appreciably change the length of path of the flute upon a screw member of given length, nor does it'change the angle of the flute with respect to a longitudinal element of the screw member. The limiting ratio of one to three provides for an adequately long path upon a screw member of reasonable length, and also avoids the loss of efliciency which results when this angle is too small.

The diameter of the screw member will, for practical reasons, be ordinarily between about one inch and three inches. For reasons of rigidity and torsional strength it must not be too small, whereas on the other hand there is no point in making it too large, since the corresponding increase in peripheral speed, at a given rotational speed, tends to increase the intensity of friction upon and within the molten polymer and thus to cause objectionable overheating.

A combination of the narrowest feasible flute (stated above to be about 0.125 inch). upon a screw member of the smallest feasible diameter (stated above to be one inch), represents a ratio, of width of flute to diameter of screw, of one to eight. But in general, particularly on screws of larger diameters, the ratio may permissibly be as low as one to ten.

Thus the width of the flute is conveniently expressed in terms of the diameter of the screw member, as being between one tenth and one third of that diameter. member of diameter 1.625 inches, such as that described above, the preferred range of width of flute, i. e., 0.25 to 0.50 inch, lies within this somewhat broader range.

The pitch'of the screw, ordinarily, will be automatically determined by the width of the flute employed because'the width of the land'. between successive convolutions of the flute will be made as narrow as efllciency in sealing between convolutions of the flute and acceptable resistance to wear will permit; increasing the In the case of a screw width of the land beyond this point would be simply uneconomical as it would serve no good purpose. It has been found that a width of .050 for the land" is, sufllcient both to give adequate resistance to wear and to seal the flute eflectively.

If auxiliary pumping means such as a gear pump is to be interposed between the screw pump and the spinneret, a feasible but not preferred embodiment of the invention, for the purpose of furnishing the desired pressure for extruding the molten polymer through the spinneret, more leeway is permitted-in the design of the screw pump because the screw pump is then only required to compress the,molten polymer and dissolve the entrapped gases, not to deliver the polymer at a pressure high enough to extrude it through the spinneret. Also,'-at the intake end of the screw pump, the dimensions of the flute in the screw need not be within the limits discussed above although it is preferred that they should be.

With respect to other parts of the apparatus shown in Fig. 1, various modifications without departing from the scope of the invention will be apparent to those skilled in the art. The heat-- ing chamber ll maybe widely varied in design providing only that it is adapted to effect the melting of the polymer at atmospheric pressure while the polymer is completely blanketed with carbon dioxide gas. Connecting the heating jackets l5 and 20 is obviously a matter of con- .venience since independent heating jackets would function just as, well.

The specific construction of the spinneret 30 forms no part of the present invention. The construction of these spinnerets is well known in the art and the selection of a suitable one will be based on considerations not involving the present invention.

The present invention has been described in connection with the melting and spinning of synthetic linear condensation polymers of which.

' said chamber to blan polyhexamethylene adipamide is a prominent:

specific member. Actually, the invention is applicable to the melt-spinning of any organic filament-forming composition which is subject to the formation of bubblesunder the conditions existing during its melting and spinning. The synthetic linear condensation polymers include synthetic linear polyamides, that is, synthetic linear polymers containing -CONH,- units in the linear'chain, synthetic linear polymers such as polyesters, polyethers, polyacetals, and mixed polyester-polyamides, such as may be prepared by condensation reactions as described in-United States Patent 2,071,250.

The filament-forming material used in accordance withthe present invention may contain modifying agents, e. g., luster-modifying agents, plasticizers, pigments, dyes, antioxidants, resins. et cetera. The present invention may be used to advantage in the handling of compositions in which the formation of bubbles may be caused by the presence of a modifying agent.

The invention provides a method and an apparatus whereby organic filament-forming compounds, and compositions comprising them, can be spun into coarse filaments of uniform diameter, free from bubbles and discoloration. The appartus is simple, and the relatively low pressures involved make for economy of construction and safety in operation. Furthermore, the invention makes feasible the use of polymer of high content of moisture.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.

I claim: v

1. Process of preparing coarse filaments of at least .020" diameter from a synthetic linear condensation polymer, which comprises melting said polymer while blanketed with carbon dioxide gas under a pressure not exceeding atmospheric pressure, introducing the molten polymer into a screw pump having a capacity at its intake end several times greater than its capacity at its discharge end, and delivering said molten polymer under pressure to a spinneret to cause said molten polymer to be extruded therethrough in the form of a filament.

2. Process of preparing coarse filaments of at least .020" diameter from a synthetic linear condensation polymer, which comprises melting said polymer while blanketed with carbon dioxide gas under a pressure not exceeding atmospheric pressure, introducing the molten polymer into a screw pump having a capacity at its intake end at least four times as great as its capacity at its discharge end, and delivering said molten polymer under a pressure of about 75 pounds per square inch to a spinneret to cause said molten polymer, to be extruded therethrough in the form of a filament.

3. Apparatus for preparing coarse filaments from a synthetic linear which comprises a chamber for melting said polymer, means for heating said chamber, means for introducing an inert gas heavier than air into I ket at least the lower portion thereof, a screw pump disposed below said chamber and having its intake end communicating with the bottom of said chamber, and a spinneret in communication with the discharge end of said screw pump, said screw pump having a capacity at its intake end at least four times as great as its capacity at its discharge end and comprising a screw provided at the discharge end with a flute between .050" and .150" in depth and .25" and .50" in width.

4. Apparatus for preparing coarse filaments from a synthetic linear condensation polymer, which comprises a chamber open to the atmosphere for melting said polymer, means for heating said chamber, means for introducing an inert gas heavier than air into said chamber to blanket at least the lower portion thereof, a screw pump disposed below said chamber and having its intake end communicating with the bottom of said chamber, and a spinneret in communication with the discharge end of said screw pump, said screw condensation polymer,

pump having a capacity at its intake end at least four times as great as its capacity at its discharge end and comprising a screw between 1.0" and 2.0" in diameter and provided at the discharge end with a flute between .050" and .150" in depth and .25" and .50" in width.

5. Apparatus for preparing coarse filaments from a synthetic linear condensation polymer, which comprises a chamber open to the atmosphere for melting said polymer, means for heating said chamber, means for introducing an inert gas heavier than air into said chamber to blanket at least the lower portion thereof, a screw pump disposed below said chamber and having its intake end communicating with the bottom of said chamber, and a spinneret in communication with the discharge end of said screw pump, said screw pump having a capacity at its intake end at least four times as great as its capacity at its discharge end and comprising a screw between 1.0" and 2.0" in diameter and provided at the discharge end with a flute between .050" and .150" in depth and between one-tenth and one-third of the diameter of the screw in width.

REUBEN T. FIELDS. 

